We investigated an outbreak of measles in the winter of 1989 in a fully vaccinated school population in suburban Harris County, Texas. A total of 71 cases occurred in the high school (attack rate 3.2%). The absentee rate at the high school rose to 18% during the outbreak. The highest attack rate was among students vaccinated in the first year of life (5.3%) and diminished significantly among students vaccinated at an older age. Students last vaccinated 13 to 14 years before the outbreak had the highest attack rate, with a significantly lower attack rate in students last vaccinated with a shorter and lower interval since since last vaccination. Forty-three (18%) of 239 sera collected during the outbreak just prior to revaccination from students attending the three schools scored as negative for measles antibody by ELISA. Extrapolation from sera tested both by ELISA and plaque neutralization indicated that 8.4% of the student population was not protected against illness and as many as 53% were not protected against reinfection. Twentyfour students whose ELISA score was initially negative were reevaluated 9 to 10 months after the booster vaccination and 20 developed neutralizing antibodies at levels protective against illness, but only 5 developed neutralizing antibodies at levels protective against reinfection. The immediate impact of revaccination on elementary and high school age students in this community for measles was to reduce the percentage of students unprotected against illness from 8.4% to 4.5% and the percentage unprotected against reinfection from 53% to about 40%. The revaccination program for elementary and high school age students had little impact on the portion of students susceptible to illness or reinfection without clinical illness.